The present invention relates to coated cemented carbide inserts with a binder phase enriched surface zone and processes for the making of the same. More particularly, the present invention relates to coated inserts in which the binder phase enriched surface zone has been modified in such a way that a unique combination of toughness behavior and plastic deformation resistance can be achieved.
Coated cemented carbide inserts with binder phase enriched surface zone are today used to a great extent for machining of steel and stainless materials. Through the use of a binder phase enriched surface zone, an extension of the application area for such inserts is obtained.
Methods of producing binder phase enriched surface zones on cemented carbides containing WC, cubic phase and binder phase are known as gradient sintering and have been known for some time, e.g., through U.S. Pat. Nos. 4,277,283, 4,610,931, 4,830,930 and 5,106,674.
U.S. Pat. Nos. 4,277,283 and 4,610,931 disclose methods to accomplish binder phase enrichment by dissolution of the cubic phase close to the insert surfaces. Their methods require that the cubic phase contains some nitrogen, since dissolution of cubic phase at the sintering temperature requires a partial pressure of nitrogen (nitrogen activity) within the body being sintered exceeding the partial pressure of nitrogen in the sintering atmosphere. The nitrogen can be added through the powder and/or the furnace atmosphere at the beginning of the sintering cycle. The dissolution of cubic phase results in small volumes that will be filled with binder phase giving the desired binder phase enrichment. As a result, a surface zone generally about 25 .mu.m thick consisting of essentially WC and binder phase is obtained. Below this zone, a zone with an enrichment of cubic phase and a corresponding depletion in binder phase is obtained. As a consequence, this zone is embrittled and cracks grow more easily. A method of elimination of this latter zone is disclosed in U.S. Ser. No. 08/019,701 (our reference: 024000-927), herein incorporated by reference.
Binder phase enriched surface zones can also be formed by controlled cooling, e.g., according to U.S. Pat. No. 5,106,674, or by controlled decarburization at constant temperature in the solid/liquid region of the binder phase after sintering or in the process of sintering, e.g., according to U.S. Pat. No. 4,830,930. The structure in this kind of binder enriched cemented carbide insert is characterized by an up to 25-35 .mu.m thick surface zone containing stratified layers, 1-3 .mu.m in thickness, of binder phase mainly parallel to the surface. The thickest and most continuous layers are found close to the surface within the first 15 .mu.m. Furthermore, the interior of the insert is characterized by a certain amount of free carbon.
The ability of certain cemented carbides to form a stratified structure has been known for a long time. The degree of binder phase enrichment in the zone and its depth below the surface depend strongly on the interstitial balance and on the cooling rate through the solidification region, after sintering. The interstitial balance, i.e., the ratio between the amount of carbide/nitride-forming elements and the amount of carbon and nitrogen, has to be controlled within a narrow composition range for controlled formation of the stratified layers.
Cemented carbides with a binder phase enrichment formed by dissolution of the cubic phase are normally characterized by, in comparison with stratified ones, a rather low toughness behavior in combination with a very high plastic deformation resistance. The comparably low toughness level and high deformation resistance shown by this type of cemented carbides are largely due to the enrichment of cubic phase and the corresponding binder phase depletion in a zone below the binder phase enriched zone.
Cemented carbides containing stratified binder phase gradients are normally characterized by extremely good toughness behavior in combination with somewhat inferior plastic deformation resistance. The toughness behavior is a result of both the binder phase enrichment and the stratified structure of the binder phase enrichment. The reduced plastic deformation resistance is to the dominating part caused by local sliding in the thick binder phase stratified layers closest to the surface due to the very high shear stresses in the cutting zone.